linux-kernel - Re: [PATCH 1/1] tracing: Simplify trace_array

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Date:	Fri, 19 Jul 2013 12:26:57 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] tracing: Simplify trace_array_get()

On Fri, 2013-07-19 at 17:51 +0200, Oleg Nesterov wrote:
> trace_array_get() scans the global ftrace_trace_arrays list
> to ensure that "this_tr" was not removed by instance_delete().
> 
> This looks a bit confusing, we can simply use list_empty() with
> the same result. list_empty() == F can not be false positive,
> new_instance_create() does everything under the same lock and
> tracer_alloc_buffers() which plays with global_trace is __init.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/trace/trace.c |   12 ++++--------
>  1 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 0cd500b..ee471fb 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -206,16 +206,12 @@ LIST_HEAD(ftrace_trace_arrays);
>  
>  int trace_array_get(struct trace_array *this_tr)
>  {
> -	struct trace_array *tr;
>  	int ret = -ENODEV;
>  
>  	mutex_lock(&trace_types_lock);
> -	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> -		if (tr == this_tr) {
> -			tr->ref++;
> -			ret = 0;
> -			break;
> -		}
> +	if (!list_empty(&this_tr->list)) {

Because this_tr can be freed outside the lock. Accessing this_tr->list
can cause a crash.

The point of the search is, if it's on the list, it will not be freed.

-- Steve


> +		this_tr->ref++;
> +		ret = 0;
>  	}
>  	mutex_unlock(&trace_types_lock);
>  
> @@ -6019,7 +6015,7 @@ static int instance_delete(const char *name)
>  	if (tr->ref)
>  		goto out_unlock;
>  
> -	list_del(&tr->list);
> +	list_del_init(&tr->list); /* trace_array_get() checks list_empty() */
>  
>  	event_trace_del_tracer(tr);
>  	debugfs_remove_recursive(tr->dir);


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